For the past several years, MEMS structures have been playing an increasingly important role in consumer products. For example, MEMS devices, such as sensors, detectors and mirrors, can be found in products ranging from air-bag triggers in vehicles to displays in the visual arts industry. As these technologies mature, the demands on precision and functionality of the MEMS structures have escalated. For example, optimal performance may depend on the ability to fine-tune the characteristics of various components of these MEMS structures. Furthermore, consistency requirements for the performance of MEMS devices (both intra-device and device-to-device) often dictate that the processes used to fabricate such MEMS devices need to be extremely sophisticated.
MEMS resonators are also becoming more prevalent. For example, a clocking device for an integrated circuit (IC) may be based on a MEMS resonator. However, if not compensated, the resonance frequency of the resonator member in such a MEMS structure may vary with temperature. Thus, a MEMS resonator may be unable to meet the specifications for frequency stability over the range of ambient temperatures needed for a particular application or may be unreliable for use in devices that generate varying levels of heat during operation. FIGS. 1A-B illustrate isometric views representing a MEMS structure having a non-compensated resonator member responding to an increase in temperature, in accordance with the prior art.
Referring to FIG. 1A, a MEMS structure 100 comprises a resonator member 104 attached to a substrate 102. Substrate 102 is comprised of a material having a first coefficient of thermal expansion (CTE1) and MEMS structure 100 is comprised of a material having a second coefficient of thermal expansion (CTE2), where CTE2 is greater than CTE1. The material of MEMS structure 100 and, hence, resonator member 104 also has a negative thermal coefficient of frequency (TCf). That is, the resonance frequency of resonator member 104 decreases in response to an increase in temperature. As depicted by the arrows in FIG. 1B, the CTE mismatch (CTE2>CTE1) results in a compressive stress induced on resonator member 104 in response to an increase in temperature. That is, substrate 102 effectively constrains the expansion of resonator member 104. This compressive stress may exacerbate the already decreasing frequency tendency of resonator member 104 from the negative TCf.
Temperature-compensated resonator members have been fabricated by using multiple materials with differing physical characteristics. For example, silicon dioxide has been used to control the TCf of poly-SiGe resonators. However, the incorporation of additional materials into a resonator member may impact the high-Q and the high stability normally associated with the single material counterparts.